Home > Ephemeris Program, Twilight > 08/29/2014 – Ephemeris – Twilight is shorter now than it was in June

08/29/2014 – Ephemeris – Twilight is shorter now than it was in June

August 28, 2014

Ephemeris for Friday, August 29th.  The sun will rise at 7:01.  It’ll be up for 13 hours and 22 minutes, setting at 8:24.   The moon, half way from new to first quarter, will set at 10:09 this evening.

Here we are at the end of August already.  We have one more night to view the Milky Way in darkness after the moon sets, because the Moon sets at the very end of astronomical twilight.  By the way, astronomical twilight starts and ends when the sun is 18 degrees below the horizon, and the actual twilight glow is completely gone.  The sun sets more than an hour before it did in late June, which means that it gets dark much earlier.  Twilight lasted much longer in June than it does now because the Earth’s rotation now drops the sun below the horizon at a steeper angle.   As a matter of fact while the sun sets an hour earlier now than in June, twilight ends two hours earlier.  It sneaks up on you if you’re not paying attention to it.

Times are for the Traverse City/Interlochen area of Michigan.  They may be different for your location.

Addendum

Tonight's end of astronomical twilight

Distance the Sun must travel from the horizon to 18 degrees below tonight, August 29, 2014. Created using Stellarium.

 Solstice end of twilight

Distance the Sun must travel from the horizon to 18 degrees below, the night after the June solstice, June 22, 2014. Created using Stellarium.

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  1. richard Kuschell
    August 29, 2014 at 2:24 pm

    At an outreach I was asked from what altitude is the 18* measured. I said sea level but was told that was maritime are they the same?

    • August 29, 2014 at 2:42 pm

      richard Kuschell:

      At an outreach I was asked from what altitude is the 18* measured. I said sea level but was told that was maritime are they the same?

      The horizon can be defined as 90 degrees from the zenith. It’s what a bubble level points to when it’s level. The sea horizon is something else again.

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